1. Filed of the Invention
The present invention relates to an optical attenuator, and particularly to an optical attenuator that uses a metal-doped optical fiber.
2. Description of the Related Art
FIG. 1 shows one example of an optical attenuator of the prior art in which a metal thin-film is used. In this device, metal evaporation film 7 is sandwiched and bonded between the junction surfaces of two ferrules 5 that are ground to an angle. Two optical fibers 6 are received in the ferrules. Light that is propagated through optical fiber 6 is attenuated by metal evaporation film 7.
An attenuator for optical fiber made up of high-loss optical fiber inserted between two optical fibers is described in Japanese Patent Publication No. 19001/86 (Yoshinaga et al.). The core of high-loss optical fiber composed of multicomponent glass is colored by exposure to radiation. Light propagated through the optical fiber is attenuated by passage through the high-loss optical fiber.
The attenuation capability of the attenuator generally diminishes with time because the number of color centers that are colored by exposure to radiation diminish with time. Yoshinaga et al., however, have succeeded in producing a core having a stabilized attenuation capability by subjecting the core to a heat treatment at 120.degree. C., whereby the number of color centers does not change.
An optical attenuator using high-concentration optical fiber in which a transition metal is doped is described in Japanese Utility Model Laid-open No. 15002/93 (Iwano et al.). The absorption loss of a high-concentration optical fiber is 100,000 dB/Km or more.
The above-described optical fiber is of a type in which the necessary attenuation is obtained by adjusting the amount of doped metal in the core. Normally, the metal-doped fiber used in an optical attenuator is worked to a particular fixed length. Thus, in this type of optical attenuator, either the amount of metal doping or the amount of exposure to radiation must be adjusted so as to obtain a prescribed level of attenuation within a determined limit of the length of the metal-doped fiber.
An optical attenuator of the prior art that uses the above-described metal evaporation film not only is composed of a large number of parts, but also requires oblique grinding of the surfaces between which the metal evaporation film is interposed. Such an attenuator is therefore particularly time-consuming to assemble and difficult to produce with a high accuracy of attenuation. In addition, any discrepancy between the axes of the optical fibers is difficult to adjust.
The drawbacks of an optical attenuator using metal-doped fibers include not only the difficulties of adjusting the amount of doping of the transition metal and the doped region, but also the accompanying high costs. There is also the problem that the accuracy of attenuation obtained by the device of this type exhibits variation on the order of .+-.1.5 dB. This problem originates from the structure of this type of optical attenuator that a desired attenuation of an optical fiber core of a certain fixed length is obtained only by adjusting the amount of metal doping to the core.
Providing the desired value of attenuation in this type of optical attenuator necessitates the preparation of optical fibers in advance having amounts of metal dopant that vary according to required levels of attenuation, and then when using the optical attenuator, using the optical fiber that has an attenuation close to the desired level of attenuation. Nevertheless, achieving a desired value with accuracy by varying the amount of metal dopant is generally extremely difficult.
The present invention has the object of providing an optical attenuator having a desired attenuation with accuracy by a simple method, thereby realizing an improvement in the accuracy of attenuation amount as well as an improvement in productivity.